This invention relates to electrochemical cells for controlling the properties of superconducting materials.
Ceramic oxide superconducting materials are known having critical or transition temperatures of approximately 90.degree. K. An example is Ba.sub.2 YCu.sub.3 O.sub.7-x. The origins of superconductivity in these copper-containing oxides with perovskite-related structures are not presently well understood. The reported irreproducibility of superconducting properties and the evanescence of high temperature superconductivity reported at temperatures as high as 500.degree. C. indicate that these materials are, at least in some respects, unstable or metastable with regard either to the phase or to the type of defects that are required for high temperature superconductivity. There is speculation that controlling the effective valence of copper ions in the material is important for maintaining the superconducting state. Empirical observations that the critical temperature for the onset of superconductivity, designated T.sub.c, depends upon the processing history of the sample, in particular, on the oxygen potential of the atmosphere in which the sample was prepared supports this speculation. It is known that in certain classes of oxide materials the valency of cations such as copper can be altered by variation of the chemical potential of oxygen.
The chemical potential of oxygen, or oxygen potential, can be controlled by equilibrating a solid sample with a gas phase of fixed composition during processing. This process is termed chemical control of the oxygen potential in the solid crystal. It is also known to establish the oxygen potential during processing of superconducting materials electrochemically by contacting the solid sample with a suitable electrolyte and setting the electrical potential of the sample with respect to a counterelectrode also in contact with the same electrolyte. Both of these known processes establish oxygen potential only during processing at temperatures much higher than the critical temperatures. Thereafter (at low temperatures) the material was left to interact with its service environment. Consequently, deviation in stoichiometry, phase instability, or instability of crystal defects such as oxygen vacancy arrays which are conducive to high temperature superconductivity is possible. A suboptimal composition would result and transition temperature would suffer. Thus, because these materials appear to be unstable or metastable, the best superconducting properties themselves may be lost since the prior art contemplated no way for actively controlling oxygen potential or cation valency during superconducting operation. Other superconducting materials with lower transition temperatures are also known. An example is a refractory metal nitride such as niobium nitride.